This application is an improvement in the invention disclosed in copending and commonly assigned application Ser. No. 175,479 U.S. Pat. No. 4,419,759 entitled "Concurrent Carrier Clock Synchronization for Data Transmission System", filed Aug. 5, 1980. The subject matter of said application No. 175,479 is incorporated by reference as if fully and completely disclosed herein.
The present invention is directed to the concurrent carrier recovery and clock synchronization from a double side band-suppressed carrier (DSB-SC) signal without the use of phase-lock loops (PLL's) for carrier or clock recovery.
In data transmission systems, information is transmitted in the form of modulated wave forms which can only be demodulated with the help of a coherent carrier reference signal and a clock signal synchronized to the symbol timing of the modulated waveform. A number of different techniques have been used for recovering the carrier and clock from the received signal, including both series and parallel-type recovery circuits. In a series-type recovery circuit, a coherent carrier is first established in order to remove the data from the received signal, and the data is then used to recover a synchronized clock. The time required to perform these operations successively, however, is generally unacceptable in systems operating in the burst mode.
Parellel-type systems are somewhat faster and require less "overhead" to be transmitted for the purpose of deriving carrier phase and symbol timing synchronization. However, parallel-type recovery systems suffer from their own disadvantages such as reduced S/N, increased steady state phase jitter and error rate and acquisition times which are also not entirely satisfactory.
An improved type of concurrent carrier and clock synchronization technique is disclosed in said copending application No. 175,479 wherein PLLs are used in the respective carrier and clock recovery networks, and the PLLs are cross-coupled in an interdependent recovery structure to enable a more effective clock and carrier regeneration. While this system provides significant advantages with respect to previous techniques, it does employ PLLs in its recovery networks. While these may provide a high degree of accuracy, there are regions of initial phase offset which occur frequently enough in incoming bursts in a PSK/TDMA system to render the aquisition period unacceptably long for higher data rates.